nixos/tests/virtualbox: remove mknod /dev/vda1 call

With devtmpfs introduced in 0d27df280f7ed502bba65e2ea13469069f9b275a it is
created automatically.

See https://github.com/NixOS/nixpkgs/pull/32983#issuecomment-353703083

nixos/tests/virtualbox.nix

···
   } ''
     ${pkgs.parted}/sbin/parted --script /dev/vda mklabel msdos
     ${pkgs.parted}/sbin/parted --script /dev/vda -- mkpart primary ext2 1M -1s
-    . /sys/class/block/vda1/uevent
-    mknod /dev/vda1 b $MAJOR $MINOR
-
     ${pkgs.e2fsprogs}/sbin/mkfs.ext4 /dev/vda1
     ${pkgs.e2fsprogs}/sbin/tune2fs -c 0 -i 0 /dev/vda1
     mkdir /mnt